1. Field of the Invention
This invention relates to an electrostatic copying apparatus and its constituent elements.
2. Description of the Prior Art
Recently, electrostatic copying apparatuses of the visible image-transfer type have gained widespread commercial acceptance. This type of electrostatic copying apparatus performs a copying process which comprises forming on a photosensitive member a latent electrostatic image corresponding to the image of an original document to be copied, applying toner particles to the latent image to develop it to a visible image, and transferring the visible image to a receptor sheet. The apparatus is provided with a photosensitive member which is disposed on the surface of a rotary drum or an endless belt-like member mounted within a housing and is adapted to be moved through a predetermined endless moving path (i.e., a circular or otherwise-shaped endless moving path defined by the surface of the rotary drum or endless belt-like member) according to the movement of the rotary drum or endless belt-like material, and along the moving path of the photosensitive member are located a latent electrostatic image-forming zone, a developing zone and a transfer zone in this order in the moving direction of the photo-sensitive member. In the latent electrostatic image-forming zone, corona discharge is generally applied to the surface of the photosensitive member by a charging corona-discharge device thereby charging the photosensitive member to a specified polarity. Then, by the action of an optical unit, the image of an original document placed on a transparent plate of an original-support mechanism disposed on the top surface of the housing is projected onto the photosensitive member. Consequently, the charge on the photosensitive member is selectively caused to disappear, and a latent electrostatic image corresponding to the image of the original document to be copied is formed on it. In the developing zone, toner particles are applied to the latent electrostatic image on the photosensitive member by the action of a developing device according to the charge of the latent image, thereby developing the latent image to a visible image (toner image). Then, in the transfer zone, the visible image on the photosensitive member is transferred to a receptor sheet transferred through the transfer zone, thereby forming the visible image corresponding to the image of the original document on the receptor sheet.
In order to form the desired visible image of good quality repeatedly on receptor sheets in the above-mentioned electrostatic copying apparatus of the visible image-transfer type, it is important, as well known to those skilled in the art, that the electric charge and the toner particles remaining on the photosensitive member after the transfer of the visible image in the transfer zone should be fully removed so as to avoid any adverse effects of the residual charge and toner particles on the next copying cycle. Removal of the residual charge is generally effected by exposing the entire surface of the photosensitive member to light by means of a charge-eliminating lamp, and/or by applying corona discharge to the photosensitive member by a charge-eliminating corona discharge device, after the transfer of the visible image in the transfer zone. On the other hand, the removal of the residual toner is accomplished by causing a cleaning means such as a cleaning blade or a magnetic brush mechanism to act on the surface of the photosensitive member after the transfer of the visible image in the transfer zone. When the aforesaid developing device is comprised of a magnetic brush mechanism, the developing device can be caused to function both as developing means and cleaning means.
The disadvantage with the conventional visible image transfer type electrostatic copying apparatus is that because the longitudinal size of a visible image formed on the photosensitive member does not always correspond to that of a receptor sheet, a visible image having a larger longitudinal size than the receptor sheet transferred through the transfer zone is frequently formed on the photosensitive member and makes it difficult to remove the residual charge and toner particles fully from the photosensitive member after the transfer of the visible image in the transfer zone. When the longitudinal size of the visible image formed on the photosensitive member is larger than that of a receptor sheet transferred through the transfer zone, a part of the visible image on the photosensitive member naturally remains on the photosensitive member without being transferred to the receptor sheet after the transfer of the visible image in the transfer zone. The amount of the toner particles remaining on the photosensitive member after the transfer is relatively small in that area of the visible image on the photosensitive member which has been transferred to the receptor sheet, and therefore, in this area, the residual charge and toner particles on the photosensitive member can be fully removed by the action of the suitable charge-eliminating means and cleaning means of the types mentioned hereinabove. In that area of the visible image on the photosensitive member which remains untransferred to the receptor sheet, however, a relatively large amount of the toner particles remains on the photosensitive member after the transferring operation. In this case, the light irradiated onto the surface of the photosensitive member from a charge-eliminating lamp and/or the corona discharge applied to the surface of the photosensitive member from a charge-eliminating corona discharge deivce is intercepted by the toner particles remaining in a relatively large amount, and cannot act fully on the surface of the photosensitive member, resulting in insufficient removal of the residual charge. In addition, since the remaining toner particles in this area adhere relatively firmly to the photosensitive member owing to the insufficient removal of the charge as stated above, the remaining toner particles cannot be fully removed by the aforesaid cleaning means.
In a conventional electrostatic copying apparatus of the latent electrostatic image-transfer type which differs from the aforesaid visible image-transfer type copying apparatus in that a latent electrostatic image formed on the photosensitive member is directly transferred to a copying paper without development and is developed to a visible image by application of toner particles, too, the longitudinal size of the latent electrostatic image formed on the photosensitive member does not always correspond to that of the copying paper transferred through the transfer zone, and a latent electrostatic image having a larger longitudinal size than the copying paper transferred through the transfer zone is frequently formed. In such a case, a part of the latent electrostatic image on the photosensitive member remains there without being transferred to the copying paper after the transfer of the latent electrostatic image to the copying paper, and therefore, even after the transfer of the latent electrostatic image in the transfer zone, a relatively large amount of charge remains in some area of the photosensitive member. It is not necessarily easy to remove such a relatively large amount of charge completely.
Furthermore, conventional electrostatic copying apparatus of the visible image-transfer type or the latent electrostatic image-transfer type and their constitutent elements have various problems or defects to be solved or removed as will be pointed out in the following detailed description of one embodiment of the electrostatic copying apparatus with reference to the accompanying drawings.